The Bub2p spindle checkpoint links nuclear migration with mitotic exit

Bfa1p and Bub2p are spindle checkpoint proteins that likely have GTPase act ivation activity and are associated with the budding yeast spindle pole bod y (SPB). Here, we show that Bfa1p and Bub2p bind the Raslike GTPase Tem1p, a component of the mitotic exit network, to the cytoplasmic face of the SPB that enters the bud, whereas the GDP/GTP exchange factor Lte1p is associat ed with the cortex of the bud. Migration of the SPB into the bud probably a llows activation of Tem1p through Lte1p, thereby linking nuclear migration with mitotic exit. Since components of the Bub2p checkpoint are conserved i n other organisms, we propose that the position of the SPB or mammalian cen trosome controls the timing of mitotic exit.